cmd/link: avoid exporting all symbols on windows buildmode=pie

Marking one functions with __declspec(dllexport) forces mingw to
create .reloc section without having to export all symbols.

See https://insights.sei.cmu.edu/cert/2018/08/when-aslr-is-not-really-aslr---the-case-of-incorrect-assumptions-and-bad-defaults.html for more info.

This change cuts 73kb of a "hello world" pie binary.

Updates #6853
Fixes #40795

Change-Id: I3cc57c3b64f61187550bc8751dfa085f106c8475
Reviewed-on: https://go-review.googlesource.com/c/go/+/264459
Trust: Alex Brainman <[email protected]>
Run-TryBot: Alex Brainman <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Alex Brainman <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

Author: qmuntal

misc/cgo/testcshared/cshared_test.go

@@ -401,7 +401,7 @@ func main() {
 	defer f.Close()
 	section := f.Section(".edata")
 	if section == nil {
-		t.Error(".edata section is not present")
+		t.Fatalf(".edata section is not present")
 	}
 
 	// TODO: deduplicate this struct from cmd/link/internal/ld/pe.go
@@ -418,7 +418,8 @@ func main() {
 		t.Fatalf("binary.Read failed: %v", err)
 	}
 
-	expectedNumber := uint32(2)
+	// Only the two exported functions and _cgo_dummy_export should be exported
+	expectedNumber := uint32(3)
 
 	if exportAllSymbols {
 		if e.NumberOfFunctions <= expectedNumber {
@@ -429,10 +430,10 @@ func main() {
 		}
 	} else {
 		if e.NumberOfFunctions != expectedNumber {
-			t.Fatalf("too many exported functions: %v", e.NumberOfFunctions)
+			t.Fatalf("got %d exported functions; want %d", e.NumberOfFunctions, expectedNumber)
 		}
 		if e.NumberOfNames != expectedNumber {
-			t.Fatalf("too many exported names: %v", e.NumberOfNames)
+			t.Fatalf("got %d exported names; want %d", e.NumberOfNames, expectedNumber)
 		}
 	}
 }

src/cmd/go/go_test.go

@@ -9,6 +9,7 @@ import (
 	"debug/elf"
 	"debug/macho"
 	"debug/pe"
+	"encoding/binary"
 	"flag"
 	"fmt"
 	"go/format"
@@ -2129,6 +2130,38 @@ func testBuildmodePIE(t *testing.T, useCgo, setBuildmodeToPIE bool) {
 		if (dc & pe.IMAGE_DLLCHARACTERISTICS_DYNAMIC_BASE) == 0 {
 			t.Error("IMAGE_DLLCHARACTERISTICS_DYNAMIC_BASE flag is not set")
 		}
+		if useCgo {
+			// Test that only one symbol is exported (#40795).
+			// PIE binaries don´t require .edata section but unfortunately
+			// binutils doesn´t generate a .reloc section unless there is
+			// at least one symbol exported.
+			// See https://sourceware.org/bugzilla/show_bug.cgi?id=19011
+			section := f.Section(".edata")
+			if section == nil {
+				t.Fatalf(".edata section is not present")
+			}
+			// TODO: deduplicate this struct from cmd/link/internal/ld/pe.go
+			type IMAGE_EXPORT_DIRECTORY struct {
+				_                 [2]uint32
+				_                 [2]uint16
+				_                 [2]uint32
+				NumberOfFunctions uint32
+				NumberOfNames     uint32
+				_                 [3]uint32
+			}
+			var e IMAGE_EXPORT_DIRECTORY
+			if err := binary.Read(section.Open(), binary.LittleEndian, &e); err != nil {
+				t.Fatalf("binary.Read failed: %v", err)
+			}
+
+			// Only _cgo_dummy_export should be exported
+			if e.NumberOfFunctions != 1 {
+				t.Fatalf("got %d exported functions; want 1", e.NumberOfFunctions)
+			}
+			if e.NumberOfNames != 1 {
+				t.Fatalf("got %d exported names; want 1", e.NumberOfNames)
+			}
+		}
 	default:
 		panic("unreachable")
 	}

src/cmd/link/internal/ld/lib.go

@@ -1320,9 +1320,6 @@ func (ctxt *Link) hostlink() {
 		case objabi.Hdarwin, objabi.Haix:
 		case objabi.Hwindows:
 			argv = addASLRargs(argv)
-			// Work around binutils limitation that strips relocation table for dynamicbase.
-			// See https://sourceware.org/bugzilla/show_bug.cgi?id=19011
-			argv = append(argv, "-Wl,--export-all-symbols")
 		default:
 			// ELF.
 			if ctxt.UseRelro() {

src/runtime/cgo/gcc_windows_386.c

@@ -9,6 +9,7 @@
 #include <stdio.h>
 #include <errno.h>
 #include "libcgo.h"
+#include "libcgo_windows.h"
 
 static void threadentry(void*);
 

src/runtime/cgo/gcc_windows_amd64.c

@@ -9,6 +9,7 @@
 #include <stdio.h>
 #include <errno.h>
 #include "libcgo.h"
+#include "libcgo_windows.h"
 
 static void threadentry(void*);
 

src/runtime/cgo/libcgo_windows.h

@@ -0,0 +1,12 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Ensure there's one symbol marked __declspec(dllexport).
+// If there are no exported symbols, the unfortunate behavior of
+// the binutils linker is to also strip the relocations table,
+// resulting in non-PIE binary. The other option is the
+// --export-all-symbols flag, but we don't need to export all symbols
+// and this may overflow the export table (#40795).
+// See https://sourceware.org/bugzilla/show_bug.cgi?id=19011
+__declspec(dllexport) int _cgo_dummy_export;